Forensic workers examine a bus where a driver was allegedly killed by a 'huntress of bus drivers' in Ciudad Juarez. Photograph: Raymundo Ruiz/AP

Two decades ago Ciudad Juárez became infamous for the murders of dozens of young women who often disappeared after leaving their jobs in assembly-for-export factories. Their raped, tortured and mutilated bodies were typically found dumped in the desert.

Two years ago the beleaguered border city topped global violence lists when rival drug cartels recruited and trained vicious killers to wage war as a misjudged crackdown generated more violence.

Today police are investigating what initial evidence suggests could be a different kind of serial killer – a middle-aged woman out to avenge past sexual crimes committed by bus drivers.

"Witnesses say she was about 40 to 50, was dressed in black and had blonde hair, but it might have been a wig," police spokesman Arturo Sandoval said of the prime suspect in the murder of two bus drivers killed as they worked on the same route on two successive days last week. Witness accounts indicate that she shot the drivers as she got off their buses.

The vengeance theory developed early on with reports that before the second murder she shouted: "You lot think you are so tough." It took off at the weekend when local media received an email signed by "Diana, huntress of bus drivers".

The mail claimed to be from a factory worker who had suffered violence from bus drivers and was fed up that nothing had been done to protect people like her.

"I am an instrument to take revenge for several women," the email said. "Society may think that we are weak, but in reality we are brave and if we are not respected we will make ourselves respected. Juárez women are strong."

While careful to point out that the mail could be a hoax, women's activists say they would not be surprised to find it was true. "Women here have been 100% disposable because of the situation of the city, the culture and the inaction of the police, and women have had enough," said Marisela Ortiz, a long time women's activist who in 2011 fled across the border to El Paso because of a series of death threats, driven home by the murders of other female activists.

Ortiz stresses the pain carried by relatives of murdered women who not only have to deal with the loss, but also find themselves constantly imagining the horrors they suffered before death. "I have worked with many victims and mothers of victims over the years who don't just want justice, they want vengeance too."

Nor is she surprised that bus drivers might be a target, given their reputation for abusing female passengers. They have also been accused of involvement in the murders of a number of women either directly, or as conduits to the victims for more powerful people presumed by many to be responsible for the phenomenon.

But Ortiz and others point out that bus drivers have also been targeted by the authorities when they need somebody to blame for crimes they cannot solve, cannot be bothered to investigate, or simply want to cover up.

"There are many well documented cases of abuse by bus drivers and the police have always ignored violence against women," said Juárez criminologist and forensic scientist Oscar Maynez. "This means they have also often been the perfect scapegoats."

But while the combination of sexual violence, bus drivers and vengeance is not new – and neither are female killers, whose presence in cartel death squads is well documented – the idea that a middle-aged woman would kill in the name of her sex is. "This would be the first case of a woman who is killing in order to get back at the patriarchal system," Maynez said. "That would be novel."

Meanwhile, police say they are working on establishing the authenticity of the email from Diana. They are also putting undercover officers on buses armed with an artist's impression of the murderer drawn from witness accounts.

None of which has done much to calm the fears of bus drivers, particularly on the route where the murders took place, where many drivers have not been turning up for work. According to local paper Diario de Juárez passengers on the route are also nervous about being caught in any crossfire.